Running out of stock can mean missed sales. Ordering too much can tie up cash in products that sit on a shelf for months.
The difficult part is knowing when to place the next order.
For a growing eCommerce business, inventory planning becomes more complicated as you add products, suppliers, sales channels, warehouses, and incoming shipments. Looking at your current stock count alone is rarely enough to tell you when another purchase order should be placed.
You also need to consider how quickly the product is selling, how long your supplier takes to deliver it, what inventory is already on the way, and whether demand is changing.
That is where eCommerce inventory forecasting becomes useful.
Instead of waiting until stock looks low, forecasting helps you use real inventory and sales information to make more informed reorder decisions.
In this guide, we will look at the main factors that affect reorder timing and how a more structured inventory process can help you avoid both stockouts and unnecessary overstock.
Why Reorder Timing Matters
Reordering inventory can sound simple: when stock gets low, buy more.
In practice, that approach can create problems because “low stock” means something different for every product.
Imagine two products both have 100 units remaining.
Product A sells 25 units per day and takes 30 days to restock.
Product B sells three units per day and can be replenished within one week.
Both products have exactly the same quantity available, but Product A needs attention much sooner.
That is why good inventory planning considers more than the number of units currently sitting in your warehouse.
A better reorder decision considers factors such as:
- Current available inventory
- Recent sales velocity
- Supplier or manufacturing lead time
- Incoming inventory
- Expected demand
- Safety stock
- Historical sales
- Existing Purchase Orders
The goal is to place your next order early enough that replacement stock arrives before you need it, without tying up more cash than necessary in excess inventory.
1. Start With Your Actual Sales Velocity
One of the most useful numbers in inventory planning is sales velocity.
Sales velocity tells you how quickly a product is actually moving.
A basic calculation is:
Units sold ÷ number of days = average daily sales
For example, if you sold 600 units of one SKU during the last 30 days:
600 ÷ 30 = 20 units per day
At that rate, 200 units of available stock represents approximately 10 days of inventory.
That information is much more useful than simply knowing you have “200 units left.”
You can also look at sales velocity across different time periods.
A 30-day average may help you understand recent demand, while a 60- or 90-day view can show whether the latest sales level is normal or unusual.
For seasonal products, you may also need to compare performance against the same period from previous seasons.
The important point is that reorder timing should start with how quickly the individual SKU is selling, rather than applying one generic stock rule to your entire catalog.
2. Know Your Real Supplier Lead Time
Sales velocity tells you how quickly inventory is leaving.
Lead time tells you how long replacement inventory takes to arrive.
Both numbers need to be considered together.
Your total lead time may include:
- Supplier processing
- Manufacturing
- Packaging
- Freight
- Customs
- Warehouse receiving
- Inspection or preparation
For example, if manufacturing takes 30 days and shipping takes another 15 days, your real replenishment time may already be around 45 days.
If you wait until only two weeks of stock remain before ordering, you may already be too late.
Consider this simple example:
Average daily sales: 20 units
Supplier lead time: 40 days
During those 40 days, you would expect to sell approximately:
20 × 40 = 800 units
That figure is often referred to as lead-time demand.
For more background on the relationship between lead time, reorder points, and safety stock, Shopify has a useful guide to reorder points and safety stock.
3. Add a Safety Stock Buffer
Forecasts are useful, but business rarely follows a perfect pattern.
A supplier may take longer than expected.
Demand may suddenly increase.
A freight delay may push a shipment back.
A product may perform better than it normally does.
Safety stock gives you a buffer for those situations.
It is inventory you intentionally keep available to reduce the risk of running out if something does not go according to plan.
For example, your expected lead-time demand may be 800 units, but you may decide to maintain another 150 units as a buffer.
Your approximate reorder point would then be:
Lead-time demand + safety stock
So:
800 + 150 = 950 units
Instead of waiting until stock is nearly gone, you would begin considering another order when available inventory approaches that level.
There is no single correct safety stock number for every product.
A fast-moving item with a long or unreliable supplier lead time may need a larger buffer than a slow-moving product that can be replaced locally within a few days.
4. Check What Inventory Is Already Incoming
Another common inventory mistake is looking only at what is physically available today.
Suppose you currently have:
500 units available
but another:
1,000 units are already on the way.
If you ignore the incoming stock, you may place another unnecessary order and create an overstock problem several weeks later.
A better inventory view should consider:
On-hand inventory + incoming inventory + expected sales before arrival
This becomes increasingly important as the number of suppliers and open Purchase Orders grows.
JungledIn’s Inventory Management tools are designed to keep real warehouse stock and incoming shipments visible alongside marketplace inventory, helping you make reorder decisions using more than one isolated stock number.
5. Review Reorder Requirements at SKU Level
Your fastest-selling product and your slowest-moving product should not have the same reorder rule.
Different SKUs can have very different:
- Sales velocity
- Manufacturing time
- Supplier reliability
- Minimum order quantities
- Seasonality
- Storage requirements
- Profit margins
A blanket rule such as “reorder everything when stock falls below 20%” can therefore create poor purchasing decisions.
Instead, assess each important SKU separately.
Fast-moving products may need earlier orders and larger buffers.
Slow-moving products may need smaller orders, less frequent replenishment, or no reorder at all.
This is also where historical sales information becomes useful.
You are not only asking:
How much stock do we have?
You are also asking:
Does this product still deserve another purchase order?
6. Watch for Products That Are Slowing Down
Inventory forecasting is not only about avoiding stockouts.
It can also help you avoid buying products that are losing momentum.
Imagine a product that used to sell 500 units per month but has gradually fallen to 220.
If you continue ordering it based on the old sales rate, stock will begin accumulating.
That can affect:
- Cash flow
- Warehouse space
- Carrying costs
- Purchasing flexibility
- Future inventory decisions
Before placing another order, compare recent sales velocity with historical performance.
Ask whether demand is:
- Increasing
- Stable
- Declining
A product that is slowing down should not automatically receive the same reorder quantity simply because that was the amount you purchased last time.
JungledIn Inventory Management also keeps SKU-level sales history so you can review how products have moved through supported sales channels and compare that activity with what is still available.
7. Consider Cash Flow Before You Reorder
A purchase can make sense from an inventory perspective and still create financial pressure.
Suppose your forecast suggests buying another $20,000 of inventory.
Demand may justify the purchase, but that does not necessarily mean the business should spend the full amount immediately.
You may also need cash for:
- Payroll
- Freight
- Marketplace fees
- Advertising
- Supplier payments
- Operating expenses
- Other products that also need replenishment
This is why inventory planning and financial planning should not operate completely separately.
JungledIn’s Business Finance module helps you review profit and loss, expenses, cash flow, accounts receivable and payable, and budgeting so inventory decisions can be viewed within the wider financial position of the business.
The question is not simply:
Can we sell this inventory?
It is also:
Can we afford to place this order right now without creating pressure elsewhere?
8. Use Purchase Orders to Keep Reordering Organized
Once you decide to reorder, the purchasing process should be documented clearly.
A Purchase Order gives you a structured record of what you are buying and from whom.
That may include:
- Vendor
- Products
- Quantities
- Cost
- Order date
- Purchase status
Without an organized purchasing process, incoming inventory becomes harder to track, especially when several suppliers and shipments are active at the same time.
JungledIn’s Business Operations tools let you maintain vendor records and create Purchase Orders, helping keep purchasing information connected to the rest of your operation.
That becomes especially useful as the business grows beyond a few suppliers or occasional inventory purchases.
A Simple Inventory Reorder Example
Consider a product with the following numbers:
| Metric | Example |
|---|---|
| Average daily sales | 15 units |
| Supplier lead time | 30 days |
| Lead-time demand | 450 units |
| Safety stock | 150 units |
| Approximate reorder point | 600 units |
The calculation starts with:
15 units × 30 days = 450 units
Then add your safety stock:
450 + 150 = 600 units
If the SKU approaches approximately 600 available units, it may be time to place another order.
However, that should not be the end of the decision.
You should still check:
- Is another shipment already coming?
- Has recent demand changed?
- Has supplier lead time changed?
- Is the product seasonal?
- How much will the new order cost?
- Does the business have enough available cash?
- Is the SKU still performing well enough to justify another order?
That is why good inventory forecasting goes beyond one fixed reorder number.
Manual Inventory Planning vs. Inventory Forecasting Software
Many eCommerce businesses begin with spreadsheets.
For a small product catalog, that can work.
You might track:
- Current stock
- Recent sales
- Lead times
- Reorder quantities
- Incoming shipments
- Supplier information
As the business grows, however, the spreadsheet becomes harder to maintain.
More products create more calculations.
More sales channels create additional inventory movement.
Purchase Orders need to be tracked.
Supplier lead times vary.
Warehouse adjustments change the actual stock count.
Incoming inventory needs to be considered alongside what is already available.
At that point, maintaining an accurate picture manually can become increasingly time-consuming.
Inventory forecasting software can help by bringing the relevant data into one workflow.
JungledIn’s Inventory Management includes Reorder Forecasting that uses actual sell-through velocity and manufacturing lead time to help determine when the next factory order should be placed.
It also supports operational inventory work such as warehouse stock tracking, incoming shipments, stock adjustments, barcode scanning, Purchase Order receiving, and SKU sales history.
The goal is not to remove your judgment from purchasing.
It is to give you better information before you make the decision.
Common Inventory Reordering Mistakes
Even experienced sellers can fall into a few common patterns.
Waiting Until Inventory Is Almost Gone
If your supplier needs several weeks to replenish stock, a low-stock warning may already be too late.
Using the Same Reorder Rule for Every SKU
Different products have different sales velocity, lead times, and demand patterns.
Ignoring Incoming Inventory
Open Purchase Orders should be considered before placing another order.
Ordering Based Only on Old Sales Data
Historical averages can become misleading when current demand is changing.
Buying Too Much for a Lower Unit Cost
A cheaper unit price does not automatically make a larger order financially better.
The additional stock still ties up cash and requires storage.
Ignoring Cash Flow
A profitable SKU can still create a short-term cash problem if too much money is committed to inventory at the wrong time.
Continuing to Reorder Slow-Moving Products
A product should earn its next purchase order based on current demand, not simply because it has always been reordered in the past.
Build a Reordering Process Around Real Data
There is no universal stock quantity that tells every eCommerce business when it should reorder.
The right timing depends on the product.
You need to understand:
- How quickly the SKU sells
- How much inventory is currently available
- What stock is already incoming
- How long replenishment takes
- How much safety stock you need
- Whether demand is changing
- What the business can afford to purchase
When those factors are considered together, inventory purchasing becomes much less dependent on guesswork.
For a small catalog, you may be able to calculate these numbers manually.
As your product range, suppliers, locations, and sales channels grow, a connected inventory system can make the process much easier to manage.
With JungledIn’s Inventory Management, you can track warehouse stock, incoming shipments, SKU sales history, and reorder requirements from one place.
Know What to Reorder—and When
Better inventory decisions start with better visibility.
Use actual sales velocity, lead times, incoming stock, and inventory history to decide when another order makes sense rather than waiting until stock is almost gone.
Or see how Inventory Management fits with JungledIn’s other tools for sourcing, growth, operations, and business protection.